This invention relates in general to strengthening a structure, and more specifically to a composite coating that can be applied to an existing structure in the field to increase its strength and resistance to explosive, seismic, or other forces.
Structures that must bear great weight, such as pillars, walls, or bridge spans, are often constructed from concrete. Concrete is very strong under compression, so can support its own weight as well as the weight of other structural elements, people, vehicles, and equipment.
Concrete is not strong under tension, though, and is a brittle material. Iron reinforcing rods are often embedded in concrete to increase the overall tensile strength. Even reinforced concrete needs to be very thick to withstand the forces generated by a moderate or large explosion, such as can happen in a refinery, cereal mill, power plant, or chemical plant.
Explosion forces often radiate in all directions and may change directions during the course of the blast. Thus, the forces from an explosion are not necessarily along vectors where typical load forces were expected.
Earthquakes, too, can generate large lateral forces that change direction. Many existing structures are not strong enough to withstand a large earthquake and need to be strengthened to meet current standards of safety.
Other structures that may need to withstand extreme, violent forces are the piping and tanks for holding and transporting petroleum products and other potentially explosive chemicals. Because large weights are not supported, tanks and pipes are generally not made of concrete. They are more typically made of a metal, chosen to be unreactive with the contents, nonporous, and much more ductile than concrete, such as iron, steel, or copper. The ductility of metals makes them easy to form into complex shapes, but metals, unless very thick, are generally not greatly resistant to moderate explosions. Most metals are not nearly as brittle as concrete; however, they stretch under force and eventually rupture. Extreme heat, such as can be generated by an explosion or a resulting fire, weakens most metals.
Some structures, such as water storage tanks or airplane bulkheads, were not expected to require high strength when they were designed, but are later found to need strengthening, such as to harden them against deliberate attack with explosives.
Some technology exists for strengthening structures already built, such as wrapping bridge pillars with epoxy-impregnated fiberglass panels to make them less likely to collapse in an earthquake. Some large structures can have additional concrete sprayed onto their surfaces.
Another way of rendering a building more explosion-resistant is to pile sandbags against the walls or on the roof to absorb and diffuse the forces. This is used in wartime or when an explosion is expected, as at a bomb-testing site, but is impractical for routine use and does not lend itself to all structures.
In a laboratory that uses potentially explosive chemicals, barriers of thick polycarbonate or similar material are often erected around reaction vessels. Another strategy used in laboratories and chemical plants is to include an easily knocked-down wall or roof in the design of the building. Shields and blast walls protect the personnel outside them from blasts, but do nothing to confine the explosion reactants and products to their vessel. Fire, secondary explosions, and widespread chemical contamination are common after a chemical explosion in a lab or plant and often cause more property damage and casualties than the blast itself does.
There is an increasing need for a way to strengthen structures against explosion, seismic, and other forces other than by making them extremely thick and massive. Such a means should help the structure keep its integrity, at least long enough for persons to evacuate or for hazardous materials to be removed. Preferably, such a means is applicable to complex structures and to those already built and in use.
The present invention is a composite coating that may be applied to many structures, such as buildings, bridges, storage tanks, airplane bulkheads, walls, columns, beams, and piping to increase their resistance to explosive, seismic, and other forces.
The composite consists of two layers of rubbery polymer, or elastomer, with a layer of textile embedded between the layers of elastomer. One preferred elastomer is polyurethane, which may be sprayed on as a blend of two precursor components. A mixing gun mixes the two components in the correct ratio so that the components mix in flight and begin to cure into rubbery polyurethane immediately.
A layer of mixed precursor is applied to the structure and a piece of textile is pressed onto the still tacky surface. The viscous fluid holds the textile in place. Another layer of mixed precursor is sprayed over the textile, covering it and bonding to the initial coat of polyurethane through the openings between the yarns. Each layer of cured polyurethane is in the range of 0.03 to 0.25 inch thick, with a maximum preferred total thickness of 0.5 inch. Interior corners are preferably radiused; a coving pre-cast from the same or compatible elastomer as the first and second layers may be glued into the corner to radius it before the first layer is sprayed.
The textile used is typically cloth woven of fiberglass, graphite fiber, or polyaramid (Kevlar). The textile may also be knit. The openings between yarns are in the range of 0.06 inch to 1.0 inch across. Fiber type and weave density are chosen to achieve the desired combination of elongation, stiffness, tensile strength, and cost. The weave orientation may be straight, 45xc2x0 bias, or another variation.
Multiple layers of textile may be used for some applications. An additional layer of polyurethane is applied before each layer of textile, preferably keeping the total coating thickness less than 0.5 inch.
Because the loosely-woven textile is very flexible and the elastomer is typically sprayed on, this composite coating can be used on complex shapes. The coating is thin and light-weight, making it practical even for airplane bulkheads and similar applications.
Many polymers, including polyurethane and epoxy, decompose into toxic gases when burned, so a fire-resistant paint is preferably applied as the top surface of the composite coating. In some applications, it is preferable to use an elastomer/textile combination that is inherently fire resistant, such as silicone/fiberglass.
The composite, formed-in-place coating of the present invention may be installed more quickly than wrapping with pre-impregnated panels. Quick installation lowers labor cost and is an advantage when reinforcing structures that are inhabited or in use.
The finished coating is relatively light-weight and thin, making it especially applicable to airplane bulkheads and piping. The coating of the present invention can be used where there is not enough clearance for sandbags, shields, sprayed-on concrete, or similar brute force protection.
The invention will now be described in more particular detail. Many modifications and variations of the present invention are possible; it is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.